In an Interferometric Fiber-Optic Sensor, an optical interferometer is so structured that the physical quantity to be measured induces a phase change in a beam of light traveling in an optical fiber. The phase shifted beam is caused to interfere optically with a reference beam, thereby producing a change in the intensity of the mixed beam which is proportional to the magnitude of the physical variable. Frequently, the measured quantity cannot be determined with sufficient accuracy from a direct measurement of the intensity, and further signal processing is required. A commonly used technique is to introduce an additional known time-varying modulation of the optical phase, and to employ a phase sensitive or lock-in detection scheme. While the technique of phase-sensitive detection is well known and widely applied, the implementation of the technique in practical, compact and sensitive fiber-optic sensors is presently limited by the lack of suitable modulation means. A particular case is the Fiber-Optic Sagnac Interferometer Gyroscope.
In such a device, light from a laser source is split by means of couplers into two coherent counter-propagating beams in a fiber coil. A rotation rate about an axis perpendicular to the coil induces a phase shift between the two counter-propagating beams due to the well known Sagnac effect. The counter-propagating beams interfere in the coupler nearest the coil producing a change in intensity on the detector which is proportional to the rotation rate. To implement a phase sensitive detection scheme, a modulator is placed near one end of the fiber coil to modulate the rotation induced phase difference. An oscillator which drives the modulator also provides a reference signal for the synchronous demodulation of the detector signal in a lock-in amplifier. While various modulation waveforms and frequencies are used, and various schemes for processing the resulting output signal are employed, it is a common feature of these techniques that the gyroscopic scale factor (that is, the factor relating the output signal of the gyro to the rotation rate) is directly related to the amplitude of the impressed modulation. (See e.g., R. Ulrich, "Fiber-Optic Rotation Sensing with Low Drift", Optics Letters 5, pp 173-175 (1980). Consequently, if this technique is to be used in a practical gyroscope, means for determining and controlling the magnitude of the phase modulation with an accuracy appropriate to the particular instrument is required.
In many Fiber-Optic Gyroscopes disclosed in the technical literature, phase modulation is accomplished by wrapping one or more loops of the optical fiber around a small hollow cylinder of PZT (lead zirconate titanate) piezoelectric ceramic placed at one end of the sensing coil. The cylinder is then driven in a manner to produce expansion of the circumference. The resulting stretching of the fiber modulates the optical path length and induces a relative optical phase shift between the counter-propagating beams. Use of such a PZT Modulator in a optical telemetry system has been described by D. E. N. Davies and S. Kingsley in "Method of phase modulating signals in optical fibers; application to optical telemetry systems", Electronics Letters 10, pages 21-23 (1974).
Another application was demonstrated by D. A. Jackson, R. Priest, A. Dandridge and A. B. Tveten in "Elimination of drift in a single mode optical Fiber interferometer using a piezoelectrically stretched coiled fiber", Applied Optics 19, pages 2926-2929 (1980). Application in a Fiber-Optic Gyro has been described by several authors, e.g., Ulrich, "Fiber-Optic rotation sensing with low drift", Optics Letters 5, pages 173-175 (1980).
While the PZT ceramic exhibits a large converse piezoelectric effect and therefore, provides a convenient means for producing the required stretching of the fiber, the ceramic has poor environmental stability and exhibits changes in sensitivity with temperature and aging. Hysteresis effects also are observed on cycling. These effects preclude the use of such a modulator in, for example, a gyroscopic instrument which must exhibit scale factor stability over a broad range of environmental conditions and also must have a useful life of several years, without some means for stabilizing the modulation.
Schemes for controlling the magnitude of the phase modulation have been disclosed which involve the detection and processing of the output signal at a number of the higher harmonic frequencies. (E.g., K. Bohm, P. Marten, E. Weidel and K. Petermann, "Direct rotation rate detection with a fiber-optic gyro by using digital data processing", Electronics Letters 19, pages 997-999 (1983). However, extensive complex electronics is required to implement this approach.
While use of a more well behaved piezoelectric material potentially could provide means to determine and control the modulation amplitude, the magnitudes of the converse piezoelectric sensitivity of quartz, the most stable crystal, is only about one percent that of commonly available PZT ceramic compositions.